New Disc Clutch System News - What's New In Clutches

It used to be a 500hp street car was a big deal. Today, even a stock LS9 Corvette makes 648 net horsepower straight off the showroom floor, and a real 700hp (or more) hot rod is not uncommon. The rise in streetable power and torque levels was coupled with a rise in driver expectations. "Today's customers want a clutch as smooth as stock with stock pedal pressure that will last forever and still hold 1,000 hp," says RPS Performance's R.J. Smith. Add in the hydraulic linkage used on most new cars, lighter and more compact drivetrains, and the removal of asbestos friction materials from the market, and manual transmission clutch system designers were faced with a whole new ball game.

The response has been to adapt race car technology for street use. Racers have been moving toward ever-smaller-diameter, lighter-weight discs and covers for years; taking 20 pounds out while simultaneously reducing rotational inertia significantly improves performance. According to Hays' Scott Stutler, the lighter materials "have pushed development of advanced friction materials to handle high torque loads with minimal surface area, which in turn has driven multidisc development and advances in hub design. As these technologies have become more refined, street applications have developed."

Disc CharacteristicsIt all starts with the disc; perhaps the most important part of the clutch system, it provides the friction for transferring engine torque to the drivetrain. The clutch facing or friction material is bonded or riveted to a thin metal plate on the disc. Racing clutches use a flat plate, but for street use, the plate has waves in it. Clutch makers call the wavy plate a marcel, and its purpose is to avoid chatter by smoothing clutch engagement. Street performance clutches always need at least a thin marcel.

2/16McLeod's dual-disc RST diaphragm clutch supports up to 800 hp. Changing to the more aggressive RXT facing kicks its capability up another 200 hp. A sprung floater and marceled bottom disc ensure smooth engagement for soft takeoffs away from a stoplight. The multipattern adapter plate fits just about any flywheel.

The center of the disc, called the hub, determines how harshly the clutch engages or disengages. Racing discs are typically solidly riveted to the hub for positive (but extremely harsh) engagement. Most street clutches use a coil-spring hub that allows the disc to rotate a few degrees under engagement. This softens the initial hit, reduces driveline torsional vibrations, and minimizes noise and vibration.

Disc weight is another important factor. Heavy or larger-diameter clutches put more strain on the transmission synchronizers.

Disc MaterialsThe disc's friction materials work with the pressure plate to develop the clutch's torque-transferring ability. Clutch friction materials are similar to those on brake pads (see HOT ROD, Mar. '10, "What's New in Brake Pads"). The trend is toward linings with higher friction coefficients and softer pedal pressure. "Every time you push the clutch pedal down, you are trying to push the crank out of the front of the block," points out Centerforce's Will Baty. "A heavy pedal puts stress on the thrust bearing and hydraulics."

3/16Clutch discs use a variety of different friction materials, as shown by these Hays facings, from left: organic, organic/metallic, full Kevlar, and sintered iron. The unsprung hub visible on the iron unit is an indication that it's intended for racing only.

Like brake pads, clutch friction compounds can be broadly divided into three categories: organic, sintered metal, and various exotics. Organic materials have replaced asbestos, which has been phased out because of health concerns. Some clutch makers claim no modern organic performs as well as the old-school asbestos compounds; others insist the new-gen compounds are as good or better. Nevertheless, organic formulations remain the friction materials of choice for most street and many street/strip applications. Properly formulated, they offer smooth, gentle engagement, and (depending on the exact formulation) decent heat capacity. Popular organic, nonmetallic replacements for asbestos include fiberglass, ceramic, and carbon. Kevlar or semimetallic bronze or iron compounds can be integrated into the brew to raise friction; everyone has his own recipe.

Sintered metal compounds and their relatives hold sway in racing. They can tolerate much higher heat, and the bronze, copper, iron, or full ceramic materials offer great grabbing power and a high friction coefficient. Engagement is extremely positive-good for racing but generally too harsh on the street. Metallics tend to accelerate flywheel and pressure plate surface wear. They can also be a lot heavier: A double-sided, full-face, iron disc can weigh twice as much as a comparably sized organic disc. To save weight and generate really hard engagement, some racing metallic discs use three to six separate metallic pucks on a winged disc.

7/16To fight going over center, McLeod has made subtle changes to the diaphragm fingers' shape, geometry, and pivot location. Moving the pivot point (arrow) of the fingers outboard makes for a quicker, more positive release, but it does reduce the clamping power. McLeod makes up for the loss in its choice of friction material.

Exotic alternatives gaining favor in racing and on high-dollar street cars are aerospace-quality carbon-carbon (CC) friction materials. Pure carbon molecules are cooked in an autoclave at 2,000 degrees F at 2,000 psi for six to eight months. This costs a bunch of dough, but the prices are dropping as the compounds become more popular. A corresponding carbon insert will be required on the flywheel, pressure plate, and any floater-plate surfaces.

CC fiber discs have a linear and predictable friction increase, yield great friction characteristics, are cool running, and offer significant weight savings over traditional materials. The discs are extremely light and have an ultralow wear rate. "CC never warps, glazes, or smells," RPS maintains. "Noncarbon clutches warp, leading to erratic shifts. The CC insert friction material is replaceable for life, so you never need to buy another clutch." Yet another advantage: Because CC is so light, RPS "can make an excellent nonslip clutch with a standard disc diameter for smooth engagement and long life," according to RPS.

Any of these materials may be combined to tailor the clutch characteristics for particular applications. A clutch disc can have different materials on the flywheel and pressure plate sides; or (in the case of multidisc clutches) the entire discs can be made of dissimilar materials. Examples of this approach are Centerforce's Dual Friction and McLeod's Dual Performance lines. "We take iron or bronze and sinter it to a backing plate on the flywheel side with organic material on the pressure-plate side," says McLeod's Red Roberts. "The backing plate has a marcel so it won't chatter. It's the best of both worlds."

Diaphragm Pressure PlatesPressure plates provide the force needed to engage the clutch and force the disc against the flywheel. Traditionally, the three pressure plates used have been the diaphragm, the Long style, and the Borg and Beck (B&B). Today, the diaphragm is the most popular configuration because it is light, compact, cheap to manufacture, and generally more effective at the typical rpm ranges encountered by street performance cars.

A diaphragm has some unique characteristics that make it a good choice for a wide range of applications once certain inherent design problems are overcome. Higher engine torque output requires a pressure plate assembly that develops more static load (spring pressure) to keep the clutch engaged. But at some point, the required static load at peak torque and rpm becomes so high that the clutch pedal becomes excessively stiff, putting a strain on the clutch actuating linkage and the driver's left leg.

On a coil-spring pressure plate like the Long or B&B, the pedal effort is linear, increasing as spring pressure increases. The diaphragm plate's Belleville spring has a nonlinear force curve-beyond the initial pedal tip-in point, the force required to continue depressing the pedal doesn't increase at the same rate as the load does. This characteristic makes for a much smoother actuating linkage with reasonable pedal effort, but with OEM-style flat diaphragm fingers, the pedal could go over center and stick to the floor under full-throttle at high rpm, especially if the clutch had excess travel and/or the freeplay was incorrectly adjusted.

That's why performance clutches replace old-school OEM flat-shaped fingers with a high-cone configuration. Once the pressure plate is bolted down, the fingers still maintain an upward angle, permitting the disc to disengage without over-centering the spring. Centerforce also adds centrifugal weights that float on the diaphragm. It is said to produce more force as rpm increases, preventing the diaphragm from sticking over center.

Hydraulic LinkageDiaphragm configurations are particularly suitable for the hydraulic and cable clutch mechanisms used for the last 25 years. Performance clutches for these linkage types must be carefully tailored to avoid exceeding their capacity and travel limits. On a hydraulic, if the pressure plate's load gets too high or the curve used to apply the pressure is too linear, it may exceed the system's pressure and volume capabilities. For hydraulic linkages with an integral hydraulic release bearing, the overall travel and height of the disc and pressure plate and even the finger height of the Belleville spring must match the stock unit as closely as possible to prevent the bearing from bottoming out or extending too far, all while compensating for normal clutch wear. This makes a properly designed single- or multidisc diaphragm practically mandatory.

For extreme high-power applications, it may be necessary to upgrade the entire hydraulic system to support the clutch. Upgrade or retrofit kits are available from Ram, McLeod, and other clutch manufacturers. Hays is said to be developing a booster device that alters the amount of throw at the slave cylinder without changing the clutch slave or master cylinder.

From 1979 to 2004, Ford Mustangs relied on cable-actuated clutches. Serious aftermarket clutch upgrades needed to support high-power engines also require a beefier, adjustable aftermarket quadrant to ensure proper cable functionality. McLeod will soon be releasing bolt-in hydraulic clutch conversion kits for the Fox-bodied Mustangs as well as '65 to '73 Mustangs.

Multidisc ClutchesClutch makers traditionally used a brute-force method to handle high power demands: Build a stiffer pressure plate, increase the friction coefficient of the disc, go to a larger-od disc and pressure plate, and finally move up to a coil-spring pressure plate. Accommodating the hydraulic linkage and generally smaller bellhousing envelopes of today's vehicles required clutch designers to revisit the multidisc clutch design for street or dual-purpose usage. Once perfected, it turns out that street multis work great for high-power, old-school muscle car combos as well.

11/16The springs on Centerforce's new True Sprung Hub multidisc cushion the shock and dampen vibrations on the front and rear discs. "There isn't chatter or release lag," Centerforce claims. "When driving one of our twin clutches, you would swear it was a stock single disc. It is that smooth." (This photo shows a cutaway display clutch.)

Multi-element diaphragms first became popular for oval-track racers who don't worry about standing starts or rapid power shifting. "An oval-track clutch system just flat-out needs to hold the power in terms of clamp load," Ram maintains. "Weight and diameter are more important. By using smaller-diameter multidisc clutches, we can allow the engine to accelerate and decelerate quicker. This means the racer can drive the car harder into the turns and have the engine rpm drop fast to slow the car; on acceleration out of the turn, the engine is able to spin back up to peak rpm quicker than possible with a heavier clutch." The added friction surfaces provided by multiple discs compensate in terms of holding power for the reduction in disc diameter.

In their original form, racing multidiscs aren't suitable for street use. "Multiplate, purpose-built race clutches are generally very touchy on pedal release compared with stock," says Jeff Neal at Quarter Master. "These clutches release quickly, and when you factor in the low rotating weight compared with stock, it can be tricky to drive."

Typical two- or three-disc oval-track multis come in 4.5- to 7.5-inch diameters, but to balance the needs of containing high power and improved engine acceleration with the need to avoid excessive slippage from a standing start, domestic V-8 street multis are larger, usually between 9.5 and 10.5 inches. "Two discs effectively double the amount of friction surface area," Hays says. "This means less clamp load on the plate and less aggressive friction materials can be used for a given load. Imagine a clutch that can hold 800 lb-ft but requires less pedal effort and has smooth characteristics like a single disc street/strip clutch rated to 500 lb-ft."

In the past, multidiscs sometimes experienced shifting problems in performance street and street/strip applications. This was caused by wear between the spacer posts that drive the floating metal plates between each disc, difficulty in pulling the pressure ring away from the friction discs when the clutch is disengaged, and intermediate plate warpage. These problems could cause a hung clutch or transmission synchronizer problems under hard shifting. Lighter materials, sprung hubs, strapped floaters, and other design enhancements have overcome these problems. The new Challenger, Viper, and ZR1 Corvette all come stock with a twin-disc clutch, indicating the OEMs have faith in their long-term viability.

Referencing its new Small Twin two-disc clutches, McLeod comments, "They have light pedal pressure and quick release. We made a marcel disc on the bottom, strapped the floater, and then made the top disc flat. The units work with most stock hydraulics, will hold 800 to 1,000 hp, and are easy to install. They utilize the stock flywheels and require no spacer shims."

Centerforce's new True Sprung Hub multidisc has a front disc with a sprung chrome-moly splined drive hub that's thicker than the hubs used on most multidiscs. The second (rear) disc has no hub; it aligns over the floater via six alignment pins but is actually driven by the front disc's sprung hub.

Hays is developing a one-piece dual-disc assembly containing the discs and the floater ring combined in a single, shared hub. The advantages are said to be less noise during operation, quicker engagement, and-because the clutch hub is the heaviest part of the clutch other than the cover-a 40 percent weight reduction. The compact assembly could be just what the doctor ordered for today's compact installations; a setup for a production T56, for instance, has only 4 1/2 inches to contain the pressure plate, flywheel, discs, and release bearing, including the allowable travel up and down the input shaft. Tentative availability is set for late 2010.

RPS offers twin and triple CC multis specifically tailored for selected heavily modified C5-or-newer Corvettes, V-10 Vipers, the Ford 4.6L Modular family (particularly supercharged applications like the G.T. 500), the '10 Camaro, '09 Dodge Challenger, and miscellaneous turbo imports. RPS says its CC clutches "have been a godsend for the Corvette guys. It has allowed them to slip the clutch to control their wheel speed without worrying about hurting the clutch. An '06 C5 driver recently set the unofficial six-speed quarter-mile record of 9.25 at 153 using RPS' triple-disc clutch."

So just where is the crossover point when you need to move up from a single to a multi? Although a dual disc could be run anytime (even in a stocker), consider stepping up to a multi somewhere in the 500 to 600hp range. A single disc can be made to hold 550 to 600 hp, but odds are it'll be a metallic or extremely aggressive organic with relatively high pressure plate static pressure that severely impacts daily driveability.

Coil-Spring ClutchesModern diaphragm single and multidiscs have pretty much relegated old-school coil-spring designs to the trash bin-unless you're an old-school diehard with a traditional restored Ford or Chrysler muscle car, have more than 1,200 hp on the street, or are running an upper-tier drag racer that needs a controlled-slip clutch to get the power to the ground.

Chrysler resto nuts may still run old B&B clutches, but for the rest of us, the B&B's wedged centrifugal-assist rollers are notorious for high-rpm release problems. If you do need to run a coil-spring clutch, it should be a Long-style configuration with adjustable centrifugal weights on the levers. Although single-disc clutches are still preferred for their simplicity by many bracket racers and lower-class Sportsman competitors, the refined, special drag race multidisc, slipper-style, Long variants reign supreme in higher-end drag racing. "The Long clutch is an integral part of tuning because it not only transfers power but manages power delivery," Hays proclaims. "This is crucial for max-effort 60-foot times."

Racing-style Long clutches can change the static pressure by adjusting the height of the coil springs. You can also change the levers, the lever pivot points, the counterweights, and many other clutch aspects to control how the clutch locks up as the car accelerates. "By adjusting the base pressure and counterweights, we can get the car off the line with minimal tire spin and then progressively apply the full clamp load of the clutch during the run," Ram says. "A Funny Car or Top Fuel clutch doesn't fully engage before the last 200 feet," Hays adds. Otherwise, there'd be nothing but tire smoke from those 8,000hp monsters.

Pro-level Long clutches range from three-finger configurations typically used in Pro Stock all the way up to Top Fuel 12-finger setups in which each finger has a different weight and is set at a different height. The fingers sequentially engage as the throwout bearing releases under pneumatic timer control. Clutch setup has become so critical in the Pro classes that a clutch specialist is now one of the most important team members.

Long clutch technology has also trickled down to lower classes. For example, the NMCA's 10-inch tire rule forces racers to limit power through the first 100 to 300 feet, and they do it with a Long-style slipper.

The original Long clutch pressure plates required a unique flywheel (found stock only on Ford products), but McLeod offers what it calls B&B/Long hybrid assemblies that combine the Long's superior design characteristics with the B&B's ability to bolt up to standard flywheels.

FlywheelsFlywheels are generally made from iron, nodular iron, steel, or aluminum (the latter with an insert compatible with the clutch facing). According to Quarter Master, "Billet or forged-steel flywheels are far safer than the cast ones at high rpm and can be manufactured to a target weight for either performance or driveability. We can now design and manufacture a steel flywheel to be as safe and as light as most aluminum. However, aluminum flywheels are nice for maintenance; the friction surface ring is bolted to the core, so it's an easy replacement." For serious performance, flywheels should be SFI certified.

Besides comprising part of the clutch's total friction package, flywheels act like a second harmonic damper to smooth out engine harmonics. A heavier flywheel absorbs and stores engine power. When this energy is suddenly released, it can help get a heavy vehicle moving or make a vehicle more manageable in stop-and-go driving. Conversely, a lighter flywheel lets the engine rev up more quickly once the vehicle is already in motion. Traditionally, light cars with aggressive rearend gears or those operating at sustained rpm could get away with lighter flywheels; heavier cars, street cars with mild rear gears, or Sportsman-level drag cars that needed to get off the line quickly went with the heavies. This thinking has evolved because many of today's hot rods make so much power and torque, they no longer need crutching by heavy flywheels. Unless you have a really heavy barge or mild rear gears, lean toward a lightweight flywheel above 500 hp.

15/16Ram's Pro Stock 6.5-inch-od, six-lever, three-disc, Long-style clutch has a unique lever design for complete control over the clutch's centrifugal loading. The levers were designed using solid modeling, allowing accurately predicted counterweight performance through the rpm range.

Some trick flywheels even move the weight around to better control inertia, which is greatly influenced by where the weight is placed relative to the flywheel center. The farther away the weight is from the center, the higher the inertia; the closer to the center, the lower the inertia. This means a heavier flywheel can rev up quicker than a lighter unit if its weight or mass is concentrated closer to the center because its inertia is lower.

Many flywheels come with multiple pressure plate mounting patterns so they can accommodate a variety of clutch designs, including multidiscs. For example, Hays offers dual-drilled flywheels for GM LSX crate engines that accommodate diaphragm or Long clutches.

16/16These RPS billet flywheels show how clutch makers accommodate multidiscs in the same envelope as the stock clutch without upsetting the factory's hydraulic mechanism height and travel restrictions. At left is a flywheel for the twin-disc Vette unit; at right is a triple-disc flywheel -note its deeper recess needed to preserve the stock stack height.

The FutureIn the future, exotic clutch facings and more multidisc configurations will become commonplace even on OEM applications, making them more affordable. Some production technology will even trickle up into racing: Already, we are starting to see hydraulic clutches make inroads in oval-track racing.

Computer-controlled clutches are coming. Some exotic sports cars already use them. Gear changes are initiated by a paddle shifter on the steering wheel. The computer controls the gear changes and the clutch application without a clutch pedal. The result: perfect shifts every time.

Letting the ECU shift a manual trans clutch blurs the line between manual and automatic transmissions, so it shouldn't surprise anyone if today's automatic transmission torque converter is soon replaced by computer-actuated multidisc clutches. With the ECU able to closely modulate and control clutch actuation, slippage, and lockup, overall parasitic losses would be less than with a traditional converter. Some VWs already have it, and GM is rumored to be close . . . yes, a factory stock Clutch-Turbo perhaps as early as 2012.

Can you see the NHRA tech guys scratching their heads over how to classify paddle-shift cars or factory Clutch-Turbo cars? Do they belong in an automatic class, a stick shift class, or in an entirely new class? And where does that leave the legacy guys? Will they be allowed to use the new technology?